1. Field of the Invention
The present invention relates to an illuminating apparatus used in liquid-crystal projectors etc., and a projection type display apparatus using it.
2. Related Background Art
The conventional illuminating devices for projection type display apparatus were usually constructed of a combination of arc tube 1 with parabolic mirror 19 as illustrated in FIG. 10.
In FIG. 10, white light emitted from a light-emitting portion 1c of the arc tube 1 (light source) is converted into nearly parallel light by the parabolic mirror 19 and a first lens array 3 forms a light source image of the arc tube 1 at the center of each lens of second lens array 4. The first lens array 3 and the second lens array 4 have their respective focal lengths approximately equal to each other, and the first lens array 3 and the second lens array 4 are spaced from each other so that the spacing between them is approximately equal to the focal length of the first lens array 3.
The light condensed by the first lens array 3 is separated into p-polarized light and s-polarized light by a polarization separating layer 5B of a polarization converting element 5. The s-polarized light is reflected and is further reflected by an adjacent polarization separating layer 5B, whereby the light emerges from between half wave plates 5A arranged in a reed screen pattern on the exit side of the polarization converting element 5. On the other hand, the p-polarized light passes through the polarization separating layer 5B and then through the half wave plate 5A to undergo phase conversion, whereby the direction of the polarization axis thereof is aligned with that of the s-polarized light. Therefore, all the beams emitted from the polarization converting element 5 are the polarized light having the axis of polarization along the same direction. Reference symbol 5C designates shield plates arranged in a reed screen pattern.
The light emerging from the polarization converting element 5 is condensed by first condenser lens 6 to be deflected onto display regions 8R, 8G, 8B of respective image modulating devices, each device being comprised of a liquid-crystal panel, where the light is modulated in each color of R, G, or B. Among the light emerging from the first condenser lens 6, red light is reflected by a dichroic mirror DM1 and the rest green light and blue light is transmitted thereby. The red light reflected by the dichroic mirror DM1 is guided via reflecting mirror M1 and second condenser lens 7R to the display region 8R of the image modulating device for red. The light transmitted by the dichroic mirror DM1 is separated into green and blue beams by the dichroic mirror DM2. The green light is reflected by the dichroic mirror DM2 to be guided through the second condenser lens 7G to the display region 8G of the image modulating device for green. The blue light transmitted by the dichroic mirror DM2 is condensed by third condenser lens 11 and reflected by reflecting mirror M2 to be guided through relay lens 12 and via reflecting mirror M3 and second condenser lens 7B to the display region 8B of the image modulating device for blue. In the drawing, P1 denotes polarizing plates on the entrance side and P2 polarizing plates on the exit side. The second condenser lenses 7R, 7G, 7B are placed for condensing the beam emerging from the first condenser lens 6 onto the entrance pupil of projection lens 10. For color composition, a cross dichroic prism 9 is positioned between the display regions 8R, 8G, 8B of the image modulating devices and the projection lens 10. The projection lens 10 is designed so as to be telecentric with respect to the display regions 8R, 8G, 8B of the image modulating devices, and angles of incidence at dichroic film surfaces of the cross dichroic prism 9 are arranged so as to be constant everywhere on the dichroic films, thereby preventing chromatic unevenness from occurring due to difference in the angles of incidence on the dichroic films. Beams modulated by the respective display regions 8R, 8G, 8B of the image modulating devices undergo color composition in the cross dichroic prism 9 and combined light is projected at an enlargement ratio onto an unrepresented screen by the projection lens 10.
In order to further improve the illumination efficiency, the liquid-crystal projector suggested in Japanese Patent Application Laid-Open No. 10-133141 is constructed using a light source device of a combination of an ellipsoidal mirror with a pair of lens arrays, the lens array on the light source side having the concave lens effect. An embodiment disclosed in FIG. 1 of the official gazette of this Japanese Patent Application Laid-Open No. 10-133141 has a light source section composed of a light source, an ellipsoidal mirror, a first lens array having a concave surface with the concave lens effect on the entrance side, and a second lens array, thereby realizing the smaller size of the lens arrays than those before it.
Incidentally, in the ordinary liquid-crystal projectors as illustrated in FIG. 10, it is important in order to improve the illumination efficiency that the eclipse at the shield plates 5C of the polarization converting element 5 be reduced by improving the parallelism of the beams incident to the first lens array 3 and that the eclipse at the entrance pupil of the projection lens 10 be reduced by decreasing the diameter of the whole light emerging from the polarization converting element 5.
When the parabolic mirror is used in the light source section, the focal length of the parabolic mirror, however, has to be increased in order to improve the parallelism of the beams emerging from the parabolic mirror. As a result, when a take-in angle of the light emitted from the light source is fixed at the reflector, the exit diameter of the parabolic mirror becomes larger at an increase ratio of the focal length of the parabolic mirror. Conversely, the focal length of the parabolic mirror has to be decreased in order to decrease the exit diameter of the parabolic mirror. The decrease of the focal length will degrade the parallelism of the beams emerging from the parabolic mirror when it is considered that the light source has the finite size.
As described above, the parallelism of the light emerging from the light source section, and the exit diameter are in the relation of tradeoff. With use of the parabolic mirror in the light source section, it was thus impossible to realize the light source section with good parallelism and small exit diameter while assuring a sufficient take-in angle of the light emitted from the light source.
For these reasons, the conventional example described in the aforementioned official gazette employed the ellipsoidal mirror and the concave lens (negative lens) in the light source section in order to improve the illumination efficiency, but optimization of the shape of the concave lens was not enough, though the size reduction of the lens arrays was realized to some extent; therefore, it had the problem that the illumination efficiency was not increased so much.
The present invention has been accomplished in view of the problems in the above conventional examples, and an object of the present invention is to realize the size reduction of the lens arrays and the increase of the illumination efficiency.
In order to accomplish the above object, an illuminating apparatus according to a first aspect of the present invention is one comprising a light source, a reflector for collecting light emitted from the light source, a negative meniscus lens which is convex on the light source side, a first optical element comprised of a lens array comprising a plurality of lenses, and a second optical element comprised of a lens array comprising a plurality of lenses, wherein the negative meniscus lens is laid between the light source and the first optical element.
In order to accomplish the above object, an illuminating apparatus according to a second aspect of the present invention is one arranged to collect light from a light source, direct the light toward a lens array system, and effect illumination with the light from this lens array system, wherein a negative meniscus lens which is convex on the light source side is provided in an optical path of the collected light. The lens array system is, for example, one comprising at least one fly""s eye lens or lenticular lens.
Here the negative meniscus lens means a meniscus lens having a negative refracting power (concave lens action).
In a preferred embodiment of the present invention, the reflector is an ellipsoidal surface of revolution and satisfies the following condition:
0.03 less than f1/f2 less than 0.07xe2x80x83xe2x80x83(1)
(where f1 is the first focal length of the ellipsoidal reflector and f2 the second focal length of the ellipsoidal reflector).
Condition (1) defines a ratio of the first focal length f1 to the second focal length f2 of the ellipsoidal reflector preferably used in the present invention. In the region below the lower limit of Condition (1), the second focal length f2 is too long, so that the size of the light source section becomes large. In the region above the upper limit of Condition (1), the focal length f2 is too short, so that the negative lens can interfere with the ellipsoidal reflector. Therefore, the regions off the above range are not preferable.
The negative lens is a negative meniscus lens and satisfies the following conditions:
0.5 less than xe2x88x92ff/f2 less than 0.8xe2x80x83xe2x80x83(2)
0.5 less than L/f2 less than 0.8xe2x80x83xe2x80x83(3)
(where ff is the focal length of the negative meniscus lens and L is the spacing between the negative meniscus lens and the second focal point of the ellipsoidal reflector).
Condition (2) defines a ratio of the focal length ff of the negative meniscus lens preferably used in the present invention to the second focal length f2 of the ellipsoidal mirror. In the region below the lower limit of Condition (2), the exit diameter of beams from the light source section becomes small, but the parallelism of beams is too poor, so as to decrease the illumination efficiency. In the region over the upper limit of Condition (2), the exit diameter of beams from the light source section is too large, so that a great eclipse occurs at the entrance pupil of the projection lens, thereby decreasing the illumination efficiency.
Condition (3) defines a ratio of the spacing L between the negative meniscus lens preferably used in the present invention and the second focal point of the ellipsoidal reflector to the second focal length f2 of the ellipsoidal reflector. In the region below the lower limit of Condition (3), the exit diameter of beams from the light source section becomes small, while the parallelism of beams becomes too poor, so as to decrease the illumination efficiency. In the region over the upper limit of Condition (3), the exit diameter of beams from the light source section becomes too large and a large eclipse appears at the entrance pupil of the projection lens, thus decreasing the illumination efficiency.
Basically, the beams emerging from the negative meniscus lens become parallel, if the negative meniscus lens is positioned a distance approximately equal to the focal length of the negative meniscus lens apart from the second focal point of the ellipsoidal reflector.
This negative meniscus lens is preferably one further satisfying the following condition:
xe2x88x924 less than SF less than xe2x88x921.5xe2x80x83xe2x80x83(4),
provided that SF=(R2+R1)/(R2xe2x88x92R1) (where R1 is a radius of light-source-side curvature of the negative meniscus lens and R2 a radius of image-display-side curvature of the negative meniscus lens).
Condition (4) defines the shape of the negative meniscus lens preferably used in the present invention. In the region below the lower limit of Condition (4), the radius of curvature of the lens surface on the first lens array side becomes too small, so that a loss of light amount can be caused by total reflection in the periphery of the lens. In the region over the upper limit of Condition (4), the curvature of the refractively acting surface of the negative meniscus lens is small on the first lens array side, so as to increase the exit diameter of the beams emerging from the light source section, so that the eclipse becomes large at the projection lens, thereby decreasing the illumination efficiency.
Further, the negative meniscus lens preferably comprises at least one aspherical surface.